fix(dlr): retain provider correlations for seven days

[lykakis]

Summary

• extend provider-message correlation retention from three days to seven days
• align correlation retention with the unresolved-message lifetime
• clarify the two DLR retention lifecycle stages in the documentation

Rationale

Seven days is the typical default retention window for delivery receipts. Keeping correlations for only three days prevented Sendium from resolving provider DLRs during the remaining four days that the corresponding message was retained. Aligning both records preserves correlation for the full waiting period.